Current tires are 225/75 r16 E load range

 

The best tire size is what Chevy says. Even down to the air pressure. Tire manufacturers know what the tire will handle, but Chevy knows what the vehicle needs, and what tires are right.

 

I had the same van. Mine was a 2002, but yes the factory tires were the old 225's. Gm switched the factory tire size to 245's a few years later.
Here is the skinny.
I bought my van, and it had aftermarket mags and 245 75 16.....which are taller and wider than factory. (a little over the 3% you are not supposed to exceed).
The van also came with the factory steel rims with the centre caps and beauty rings, and the factory sized 225's.
I did a mileage check with the lager tires, and got about 16 mpg gallon hwy, and 13 city.
I put the factory rims on and got slightly better city, but worse hwy.....
The larger tires have greater rolling resistance, and require more power to get going, but at cruising speed, they give you a longer final drive.....
At the end of the day, the diff is only a mile and a half to the gallon.....so not really worth spending a grand to change tire sizes.....
Your van is gonna be a pig on fuel....sorry....lol.

 

If gas milage is what you seek... and you dont do any heavy towing....
and if you plan on keeping the van for a long time.. then just change
gear ratio... for what you would spend on another set of GOOD tires..
you can invest in another set of gears... I have 3.42 in my GMC truck
and 3.73 in my van I would say you would be very happy if you went
with the 3.42 ( just an idea ) I know every one has their own
desires for their vans.